In a modern mobile network, voice traffic is handled by Circuit-Switched (CS) networks while data traffic is handled by Packet-Switched (PS) networks (e.g., data links provided by second generation (2G) data, third generation (3G) data, high speed packet access (HSPA), 1×, Long Term Evolution (LTE), LTE-Advanced (LTE-A), etc.). However, and despite the fact that PS networks were originally designed as data networks, their quality of service (QoS) and capacity gains provide subscribers and operators significant additional benefits for voice services, such as High Definition (HD) voice, enhanced video capabilities, and rich communication offerings. Central to the enablement of today's smartphones is meeting the high expectation for mobile user communication experience by augmenting voice calls and video calls with these richer media services. To that end, Media services, such as Rich Media Services, Multimedia, Voice, Video, file sharing, etc., have been introduced.
An example of Media over Data services, VoLTE is a voice telephony solution comprising the IP Multimedia Subsystem (IMS) and the multimedia telephony (MMTel) service that delivers voice services over LTE access. Although VoIP using Internet-based PS 2G/3G applications (e.g. mobile Skype) have been available since early 2010, the data streams in these “over the top” (OTT) voice applications are not differentiated from other IP data traffic, so network and user device IP traffic loads can severely compromise voice quality of service. VoLTE, in contrast, operates as a native application in the user's mobile device, enabling prioritization over other data streams to deliver a high QoS. Based on IMS/MMTel, voice services can be further enriched with video and combined with several other enhanced IP-based services such as HD voice, presence, location and Rich Communication Suite (RCS) additions like instant messaging, video share and enhanced/shared phonebooks. The introduction of VoLTE, though evolving as the mobile industry infrastructure evolves toward higher LTE availability, presents unique challenges.
One of those challenges is ensuring voice call continuity (VCC) as mobile devices move from an area with LTE coverage to a non-LTE coverage area, by handing over a voice call from the LTE PS network to a CS network, and simultaneously transition the audio stream from VoLTE packet-switched delivery to CS delivery, while the call is in progress. This is especially important since in many cases, service providers build and expand their LTE networks gradually. As a result, LTE networks and the VoLTE services built on top of them must be able to coexist with CS networks and to ensure handover to the CS network when LTE coverage is insufficient. Each VCC handover, however, increases the risk of a dropped call. This is because each VCC handover must retune the mobile device to a new frequency, must acquire and begin transmitting on the legacy network, and must transition from delivering audio packets via a packet-switched solution to a circuit-switched delivery, all in near simultaneous fashion while minimizing any disruption to the real-time voice call that is in progress. Therefore, it is desired to minimize the number of VCC handovers.
Another challenge presented by an implementation of VoLTE, although not actually inherent to the VoLTE technology, relates to the Radio Frequency (RF) uplink budget performance of VoLTE in relation to the performance of voice in legacy technologies, such as 2G or 3G networks. It has been found that VoLTE deployments can result in a shortfall on the LTE link budget that can reach 12 dB. The link budget shortfall is especially challenging when VoLTE is implemented at higher bands, since the link budget used for the Radio Frequency (RF) network design often has less margin in bands greater than 1.5 GHz, for example. Although the link budget shortfall was anticipated, actual VoLTE implementations have revealed it to be greater than expected.
The link budget shortfall has a significant impact on VoLTE performance. For example, the link budget shortfall results in a high Packet Error Rate (PER), which results in poor voice quality. Furthermore, the link budget shortfall also results in a greater number of VCC handovers, since the system recognizes that VoLTE coverage is marginal and so it switches to a fallback circuit switched protocol via VCC, which increases the risk of a dropped call.
Several solutions have been proposed to address the link budget shortfall. One solution would restrict the deployment of VoLTE to bands less than 1 GHz. However, this presents a problem because RF spectrum is limited and thus often not available in this range. Another solution that has been proposed is simply optimizing LTE for voice by use of enabling technologies such as Transmission Time Interval (TTI) bundling, antenna tilting, or terminal transmission power settings. However, some of these key VoLTE enablers are not yet available, and the impact of their implementation will not always be enough to overcome the VoLTE uplink budget issues.
Given that it appears that the majority of service providers will evolve their networks to LTE or other forms of advanced data networks, there is a need in the art for a solution to ensure a consummate user experience, while taking into considerations the challenges presented by the technology implementing Media services, such as Rich Media Services, Multimedia, Voice, Video, file sharing, etc.